The use of a plurality of modular filter elements or cells, rather than a single large filter bed, presents distinct advantages for certain water and wastewater treatment systems. Not only is there considerable reduction in the size and complexity of the backwash apparatus, but also the need for a total system shutdown during the backwash operation is removed.
In accordance with the modular design, the smaller modular elements are each cleaned individually, and therefore, smaller pumps, plumbing components and associated backwash controls can be used. Certain components, such as backwash effluent storage tanks, air scour apparatus and mechanical rakes, can be eliminated through the use of modular filter construction. Also, with only one modular filter being backwashed at any given moment, the other modular filters remain operational, providing continuous filtration capabilities for the system.
Despite its attractiveness, however, the modular filter system concept has not been exploited to full advantage owing to shortcomings in the design of associated backwashing apparatus. The present invention overcomes the prior limitations in that it employs an overhead movable carriage and platform from which a specially designed backwash hood assembly depends. The hood assembly is selectively positioned over a particular filter cell and then lowered to form a water-tight seal with that cell, hydraulically isolating the modular cell from the surrounding body of water. This approach provides effective and rapid cleansing of the filter media within an individual cell without disrupting the ongoing filtration process occurring in the remaining filter cells.
The patents to C. E. Fraser, U.S. Pat. No. 2,442,592 and C. S. Horning et al, U.S. Pat. No. 3,239,061 are illustrative of the prior art which uses a carriage or the like, suspended above an open portion of the filter tank. The powered carriage transports a backwash apparatus over successive portions of the underlying filter media. Neither patent, however, contemplates the modular filter construction in combination with the hydraulic isolation capabilities of the movable backwash apparatus of the present invention.
H. Bendel, U.S. Pat. No. 3,984,326, provides individual compartmentalized flushing but relies solely upon the upward flow of backwash water resulting from the "vacuum pressure" created by a single pump. Bendel's disclosure is devoid of any suggestion as to the advantages of a high intensity scouring action provided by filter cleansing means, such as jetwash probes, or the like.